11.5 Convective and wind-driven turbulence at mixed layer fronts

Thursday, 11 June 2009: 3:10 PM
Pinnacle BC (Stoweflake Resort and Confernce Center)
John R. Taylor, MIT, Cambridge, MA; and R. Ferrari

This study examines the influence of a horizontal buoyancy gradient on convective and wind-driven turbulence in the surface mixed layer. In the absence of a horizontal buoyancy gradient, surface buoyancy loss results in `upright' convective mixing, while a surface wind stress leads to a turbulent Ekman layer. The scaling of the mixed layer growth rate under each of these forcing types can be formulated in terms of potential vorticity which naturally includes the horizontal buoyancy gradient. Numerical simulations in an idealized geometry are used to study this problem and to test the derived scaling law. Starting with a uniformly stratified fluid and an imposed horizontal buoyancy gradient, turbulence is driven by imposing a surface heat loss or a de-stabilizing wind-stress at the upper boundary and a turbulent mixed layer soon develops. Under typical conditions, two dynamical regimes are found in the mixed layer. Upright convection is seen near the surface and at weak fronts where the buoyancy flux is the dominant source of turbulent kinetic energy, while slantwise convection is observed at strong fronts and at the bottom of deep mixed layers. The slantwise convection region is associated with a stable stratification even though the turbulent dissipation rate is large. As a result, this region could be misinterpreted in field data using criteria based on the stability of the vertical stratification and shear. Since we have found that the turbulent fluxes of buoyancy and passive scalars are significantly modified at mixed layer fronts, describing the influence of the horizontal buoyancy gradient is an important step in developing more accurate models of the mixed layer.
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